The present invention relates to variable displacement hydrostatic pump or motor units and controls therefor and more particularly relates to feedback linkages forming a portion of the displacement control means for such units.
In common usage today are hydrostatic pump or motor units including swashplates mounted for selective angular adjustment to increase or decrease the displacements of the units between zero and maximum displacement conditions. The swashplates of these units are controlled by one or more hydraulic actuators connected to the swashplates and selectively actuatable by means of control valves, which each include concentrically mounted first and second axially shiftable valve elements that are normally positioned relative to each other such as to establish a null condition in the valve for maintaining a preselected displacement condition in the pump or motor unit. The first valve element is shiftable axially relative to the second valve element in response to a command input signal such as to cause the actuator or actuators to reposition the swashplate to a new position for effecting a predetermined change in displacement corresponding to the magnitude of the command input signal. A feedback linkage is connected between the swashplate and the second valve element and acts to shift the latter to re-establish the null condition in the valve once the commanded displacement change has been effected. Two examples of such pump or motor units and controls therefor are respectively disclosed in U.S. Pat. No. 3,803,987 issued to Knapp on Apr. 16, 1974 and U.S. Pat. No. 3,810,715 issued to Week et al on May 14, 1974.
Hydrostatic pump and motor units of the general type just described are commonly coupled together to form transmissions for driving various machine parts and for driving ground wheels or tracks of vehicles. These transmissions often include a transmission case or housing which contains a pump and motor unit set and the associated control valves for the pump and motor units are mounted on the exterior of the case and coupled to the swashplate of the associated unit by a feedback linkage projecting through the case. As the case sometimes serves also as a reservoir for the hydraulic fluid used to operate the pump and motor units and the displacement control actuators, it is necessary to ensure that the interface between each control valve and the case is adequately sealed to prevent leakage of the hydraulic fluid from the case. U.S. Pat. No. 3,857,541 shows the example of how the control valve side of that part of the valve-to-case interface would have to be ported and sealed for permitting control fluid to be routed to and from the control valve.
The hydrostatic pump or motor units and controls therefor of the type described hereinabove suffer from one or more of the disadvantages of:
1. Having too much looseness in the feedback linkage resulting in inaccurate pump or motor displacement control.
2. Having a feedback linkage which occupies a relatively large amount of space and/or requires such space for its operation.
3. Having the feedback linkage connected to the control valve such that when the control valve is mounted on the exterior of a transmission case the installation and/or removal of the control valve is made somewhat difficult.
4. Having the control valve connected to the transmission case such that a large amount of fluid drains from the case when the control valve is disconnected for repairs.